The present invention is directed to ink compositions useful, for example, in thermal and acoustic ink jet printing processes. More specifically, the present invention relates to fast drying, penetrating inks with, for example, a surface tension of between about 25 to about 44 dynes/cm, and preferably between about 30 to about 39 dynes/cm, and which inks can be selected for acoustic ink jet processes, especially electric field assisted acoustic ink jet printing processes and apparatuses, reference, for example, U.S. Pat. Nos. 5,121,141, 5,111,220, 5,371,531, the disclosures of which are totally incorporated herein by reference, and the acoustic ink processes as illustrated in the above appropriate copending applications, such as an acoustic ink printer for printing images on a record medium such as size press treated plain papers, that is, for example, office papers, size press treated ink jet papers and coated pigmented papers, or coated ink jet papers.
The invention inks possess, for example, in embodiments excellent waterfastness, a surface tension of, for example, between about 25 to about 44 dynes/cm, and preferably between about 30 to about 39 dynes/cm, preferably measured at 25.degree. C., and which inks can be comprised of (1) a waterfast alkylated quaternary compound that primarily preferably maintains the surface tension of the inks at from about 25 to about 44 dynes/cm and more preferably from about 25 to about 39 dynes/cm and which compound has a molecular weight, M.sub.w of, for example, from about 105 to about 995; (2) a liquid ink vehicle that, for example, can spread the ink into the pores of a substrate, such as paper, and which vehicle preferably possesses a low acoustic loss, which enables a reduction, or minimization of energy consumption, and which acoustic loss is, for example, below or about equal to about 30 dB/mm; (3) a paper-curl reducing compound (anti-curl compound) that primarily functions to reduce, minimize, or substantially eliminate the curl of paper to about 5 to about 10 millimeters; (4) a lightfast antioxidant; (5) a water soluble organic salt, a water soluble inorganic salt, or mixtures thereof; (6) a biocide; (7) a colorant such as a dye, a pigment or mixtures thereof; and (8) an optional component, such as a diluent, like water, or other similar solvent, and preferably deionized water.
More specifically, the present invention is directed to aqueous inks comprised of (1) a waterfast alkylated quaternary compound, such as soya amidopropyl ethyldimonium ethyl sulfate, (Schercoquat, SOAS, Scher Chemicals), stearyl tributyl phosphonium bromide (Aldrich 29,303-2), cetylpyridinium chloride monohydrate, (Aldrich #85,556-1), or 1-dodecylpyridinium chloride hydrate (Aldrich #33,124-4) that, for example, maintains the surface tension of the inks from, for example, about 25 to about 44 dynes/cm, or about 30 to about 40 and preferably from about 30 to about 5 39 dynes/cm, and which compound has a molecular weight M.sub.w of, for example, from 105 to about 995; (2) a liquid ink vehicle; (3) a paper curl reducing compound, or anticurl compound that, for example, reduces the curl of paper to about 2 to about 5, or to about 3 to about 4 millimeters; (4) a lightfastness UV absorber; (5) an antioxidant; (6) a water soluble organic or inorganic salt; (7) a biocide; and (8) a colorant such as a dye, a pigment or mixtures thereof, and wherein there can be generated with such inks excellent developed images on, for example, size press treated plain papers and coated pigmented papers with acceptable image permanence, excellent projection efficiency on transparencies and excellent crease resistance, and wherein the inks possess acceptable, and in embodiments superior lightfastness of, for example, from about 90 to about 100 percent and superior waterfastness of, for example, from about 95 to about 100 percent. Moreover, in embodiments of the present invention there is enabled the elimination, or minimization of undesirable paper curl. Also, advantages of the low viscosity inks of the present invention resides in reduced power consumption by about 50 percent, thereby increased printing speed from about 25 pages per minute to about 60 pages per minute, improved jettability, that is, the drop size of the ink is uniform all across the print head allowing improved ink directionality, where the velocity of the ink drops has a standard deviation value of, for example, from about 3 to about 5 microns and consequently lower edge raggedness, such as a value of about 5 microns and preferably lower than about 3 microns. The edge raggedness, which can be measured with an optical microscope, is the deviation of the average width a line measured from the middle of the line to the edge of the line (also known as mid-frequency line-edge noise (MFLN)).